A grain-oriented electrical steel sheet is a soft magnetic material having excellent magnetic properties in the rolling direction as a result of the so-called Goss texture, in which all the grains of the steel are oriented in the {110} direction and the crystallographic orientation in the rolling direction is parallel to the <001> axis.
This grain-oriented electrical steel sheet is manufactured so as to show excellent magnetic properties by secondary recrystallized grains obtained by inhibiting the growth of primary recrystallized grains during final annealing following primary recrystallization and selectively growing grains having the {110}<001> orientation among the inhibited grains. Thus, an inhibitor of growth of the primary recrystallized grains (hereinafter referred to as the “inhibitor”) is very important. The key of the technology for manufacturing grain-oriented electrical steel sheets is that grains having the {110}<001> orientation among the inhibited grains can preferentially grow to form secondary recrystallized grains.
Secondary recrystallization in the final annealing process occurs when the inhibitor grows or is degraded with increasing temperature to loss its function to inhibit primary recrystallized grains, and in this case, grain growth occurs within a relatively short time. The growth of all primary recrystallized grains should be inhibited up to immediately before secondary recrystallization in the final annealing process occurs, and for this purpose, precipitates should be uniformly distributed in a sufficient amount and a suitable size, should be thermally stable, and should not be easily decomposed up to a high temperature immediately before second recrystallization occurs.
This {110}<001> texture can be obtained by a combination of various processes. To obtain this texture, the slab composition should be strictly controlled, and the conditions of a series of processes, including slab heating, hot rolling, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and final annealing (secondary recrystallization annealing), should be strictly controlled.
As used herein, the term “primary recrystallization” refers to general recrystallization in which new grains are nucleated and grow at a specific temperature or higher. The first recrystallization is generally performed either at the same time as decarburization annealing after cold rolling or immediately after decarburization annealing, and grains having a uniform and suitable size are formed by the first recrystallization. Generally, the orientation of grains in grain-oriented electrical steel sheets are dispersed in several directions, or orientations other than the Goss orientation have textures arranged parallel to the surface orientation, and the ratio of the Goss orientation to be finally obtained in the grain-oriented electrical steel sheets is very low.
As technologies of improving magnetic properties by controlling heating conditions in primary recrystallization annealing, those that use rapid heating in a decarburization annealing process are disclosed in Japanese Patent Laid-Open Publication Nos. 2003-3213, 2008-1978, 2008-1979, 2008-1980, 2008-1981, 2008-1982, and 2008-1983.
Japanese Patent Laid-Open Publication No. 2003-3213 discloses a technology of manufacturing a grain-oriented electrical steel sheet having high magnetic flux density by controlling the amount of nitrification and controlling the ratio of I[111]/I[411] in textures after annealing to 2.5 or less. In addition, it discloses that the amounts of aluminum and nitrogen and the heating rate in the decarburization annealing process should be controlled in order to control the textures.
Japanese Patent Laid-Open Publication Nos. 2008-1978, 2008-1979, 2008-1980, 2008-1981, 2008-1982, and 2008-1983 disclose methods of magnetic flux density by performing decarburization during hot-rolled sheet annealing or controlling the hot-rolled sheet annealing temperature to control the lamellar distance while performing rapid heating in the temperature range of 550˜720° C. at 40° C./sec or higher, and preferably 75-125° C./sec, during decarburization annealing. These patent documents disclose that {411}-oriented grains among primary recrystallized grains influence the preferential growth of {110}-oriented secondary recrystallized grains, and that grain-oriented electrical steel sheets are manufactured by controlling the ratio of {111}/{411} in primary recrystallized textures after decarburization annealing to 3.0 or less, performing nitrification and enhancing the inhibitor.
However, in these patent documents, the temperature range in which a great change in the textures is shown is 700˜720° C., and only a method of improving magnetic flux density by performing rapid heating to the temperature range of 550˜720° C. including the above temperature range (700˜720° C.) is suggested.
In addition, these patent documents have technical limitations in that they do not attempt to directly increase the ratio of grains having the Goss orientation, but attempt to increase the ratio of {411}-oriented grains that have an indirect influence on abnormal grain growth (secondary recrystallization) in the Goss orientation in secondary recrystallization annealing after decarburization annealing.
Even when the above prior patent documents are considered together, these patent documents do not suggest a method for manufacturing a grain-oriented electrical steel sheet, in which the magnetic properties of the steel sheet can be improved by controlling the density of Goss orientation in a decarburized sheet through a three-stage heating pattern of ultra-rapid heating+rapid heating+general heating (which means that the heating rate differs between temperature zones) during first recrystallization annealing.